The use of conductors arranged in pairs is well known from the use of such conductor pairs in telephone circuitry and other high-frequency applications. Such conductor pairs, and conductors generally, are used in situations where, for example, it is necessary to extend large quantities of conductors between two points. To avoid checking each conductor of a large number of conductors for conductivity at each end of a plurality of conductors, various means for identifying the individual conductors of the plurality of conductors have been devised. Among the more popular means of identifying individual conductors is through the use of a color code system, whereby individual conductors are coated with differently colored insulating material for purposes of identification. Other techniques for identifying individual conductors include the printing of markings such as stripes or words on the insulating material.
The great need for conductor pairs in applications such as those noted above has been a motivating factor in the development of a highly competitive industry in manufacturing such conductor pairs, and has led to a variety of techniques for manufacturing such conductor pairs. According to one technique, individual conductors are coated with an insulating material and subsequently bonded together by, for example, an adhesive (herein referred to as a "bonded pair"). According to another technique, individual, substantially parallel conductors are guided through an extruder apparatus in which they are both covered with insulating material in a single coating operation so that they are extruded together in an attached form or are forced together so that they attach to one another shortly after being extruded (herein referred to as an "attached pair"). The individual conductors of an bonded or attached pair are preferably separable by breaking a web or attachment point between them so that their ends can be connected to desired connectors.
It is particularly useful to be able to identify conductors as part of a conductor pair. Earlier methods of identifying conductors as part of a conductor pair included simply twisting a pair of wires together. More advantageous methods of identifying conductors as part of a conductor pair include the above-mentioned techniques for forming bonded pairs or attached pairs to avoid the likelihood of separation of conductor pairs that are not physically joined. Further, it has been found to be desirable to provide color code markings on individual ones of the conductor pairs to facilitate identification of the conductor pairs.
Industry standards such as those of the National Electrical Code.TM. and the Insulated Cable Engineers Association, Inc., (ICEA) have standardized color code schemes and established guidelines for identifying conductor pairs. For example, ICEA has promulgated Standards for Communications Wire and Cable for Wiring of Premises setting out standards for insulating material color combinations for so-called "tip" and "ring" conductors of a pair. In Section 4.4 it is stated that, "positive identification of the associated `tip` and `ring` conductors of a pair shall be accomplished by marking or striping each insulated conductor of a pair with the color of its mate". The ICEA's Standards further note that, in cables with five or less pairs, the marking of the ring conductor may be omitted. It is desirable to comply with this standard by forming a first insulated conductor that is primarily a first color and a second insulated conductor that is primarily a second color, and providing identification on one or both of the conductors, in the form of a marking or stripe of the color of the mate, so that the there is less of the colored identification on the particular conductor than there is of the primary color of the particular conductor.
Various difficulties are presented in marking or striping of tip and ring conductors in conductor pairs so that they meet industry standards. For example, conductor pairs formed as attached pairs are typically insulated with a single insulating material so that it is necessary to mark at least one conductor of the pair so that it has a mark or stripe of the color of its mate. While it is possible to provide ink markings on each conductor of the pair, beside being subject to smearing of the ink, as seem in FIG. 1A, ink markings in the form of stripes 21 are typically only applied over less than half of the conductors 23, 25 so that each conductor is more properly described as being the same color with a stripe of a different color. Ink markings in the form of bands 27 are shown in FIG. 1B are also typically applied over less than half of the conductors 23, 25 so that each conductor is properly described as being the same color with a stripe of a different color. Further, band marking of conductors is preferably performed on a single insulated conductor prior to forming a bonded pair or an attached pair, rather than in applications involving attached pairs that are extruded as attached pairs.
Another technique for applying color coding stripes is the coextrusion technique described in U.S. Pat. Nos. 2,521,123, 3,288,895, and 4,248,824. In U.S. Pat. No. 3,288,895, in which a pair of conductors is produced, a main extruder feeds the main insulating material into a main extruder die and stripes of colored material are fed into the main extruder die from auxiliary extruders to form stripes on a pair formed of another color. However, the thickness of the colored material materially alters the unit capacitance of the conductor and maintaining capacitance balance in the pair requires optimization of the depth of the stripes. Further, as shown herein at FIG. 1C, the pair of insulated conductors 23, 25 does not produce two conductors that are primarily different colors but, rather, includes two conductors that are both primarily one color, with a stripe 29 along the length of at least one of the conductors.
U.S. Pat. No. 3,720,747 to Anderson et al. describes yet another technique for color coding multiple conductors in which insulating material of desired colors is arranged in a preform, and the preform is placed in an extruder. While the technique described therein is adapted to produce pairs of conductors having primarily different colors, the technique is complicated in that it includes a number of steps, such as arranging the insulating material in the preform prior to coating of the conductor with the insulating material.
In addition to the desirability of color coding conductor pairs, it is desirable to minimize crosstalk in conductor cables. In U.S. patent application Ser. No. 08/032,149 entitled "Twisted Parallel Cable" by Kenny and Siekierka, filed Mar. 17, 1993 it is noted that it is known that twisting conductor pairs can be performed to minimize electrical coupling between paired conductors. In that application, the inventors recognize the benefits of controlling center-to-center distances in a twisted conductor pair for purposes of stabilizing capacitance within a cable. Further, the inventors recognize the benefits of controlling center-to-center conductor spacing on all cables used in a particular design so that impedances in the design are matched, thereby minimizing problems with signal loss and tracking error (jitter). The inventors propose controlling the center-to-center distance in twisted pair cables so that the variation in the distance between the centers of adjacent conductors, the center-to-center distances, along a twisted pair cable is very small. The center-to-center distance at any point along the twisted parallel cable does not vary by more than .+-.0.03 times the statistical average of at least 20 measurements of center-to-center distances taken along each 1000 feet of cable with each measurement taken at least 20 feet apart. Alternatively, or in combination, the inventors propose providing conductors and corresponding dielectric layers twisted along the length of a cable having, over any 1000 ft. length of the cable, an average impedance of approximately 90 to 110 ohms when measured at high frequencies of approximately 10 MHz to approximately 200 MHz with a tolerance of .+-.5% from an average measured from a randomly selected 1000 feet of twisted cable. It is desirable to provide a conductor pair combining the advantages of controlled center-to-center distances between conductors and/or controlled impedance characteristics proposed by Kenny and Siekierka with the simplicity of use available in color coded conductor pairs.